Fig. 4.5Diagram of some components of wall synthesis at the hyphal tip. Vesicles are thought to deliver the main wall-
synthetic enzymes (chitin synthase and glucan synthase) to the tip, where they lodge in the plasma membrane as
integral membrane proteins. Mannoproteins and other glycoproteins are transported in vesicles from the endoplasmic
reticulum–Golgi secretory system (because the glycosylation of proteins occurs only in the Golgi). Multivesicular bodies,
whose functions are still unclear, may be carried as vesicular cargoes along microtubules. Enzyme activators and inhibitors
also are thought to be involved in the orchestration of tip growth, but the substrates for wall synthesis arrive from
metabolic reactions in the cytosol.
Fig. 4.4Representation of the possible organ-
ization of wall growth at the hyphal apex. Only
half of the hypha is shown. Vesicles (V) derived
from the endoplasmic reticulum and Golgi
body (G) are transported to the apex, probably
by microtubule (M)-associated motor proteins.
The vesicles can then be directed to the plasma
membrane, perhaps by actin-associated motor
proteins. The newly formed wall at the extreme
hyphal tip is thin and has few cross-linkages, but
becomes increasingly cross-linked further back.
By contrast, the actin cytoskeleton is highly
delevoped at the extreme tip (see Fig. 3.19) and
might help to provide structural support, com-
pensating for the lack of wall cross-linking at
the tip. The concentration of actin progressively
decreases behind the tip.